Airdrops are free cryptocurrency tokens given out by developers. Usually, they are given out to create awareness for new tokens but sometimes existing tokens do give-aways.
Airdrops can be regarded as samples. Just like a company with a new product might decide to give out samples to members of the public to create awareness about such product.
These free tokens have the same worth as the ones that will be sold by the developers.
Airdrops are a good way to invest without capital. Just register for them, complete given tasks and they're are yours under specified conditions.
The conditions are usually very lenient. It could be making a tweet about the token, following an account, joining a telegram group, sharing referral links, etcetera.
Usually, you don't have to pay anything for a credible Airdrop. I have actually not come across any Airdrop that I have to pay anything for. I believe though that there will be some that would require payment. I would not recommend doing such because it defeats the purpose of the Airdrop itself.
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There are many sites that run adverts for this AIRDROPS online. If you intend to join one, just conduct a search and you should get a list. Personally, I would recommend this
Since most of them are only ERC-20 compatible you might want to get an ERC-20 compatible wallet. I won't be going into details about ERC-20 compatibility here. If you have no clue, research on it. There's tons of information available on the web if only you can try to find it.
Be smart, stay woke.